1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process and to a developing device used in the image forming apparatus. More particularly, the present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile machine, and to a developing device used in the image forming apparatus.
2. Related Background Art
In recent years, many image processing apparatuses have been proposed as a hardware unit for digital information communication through a data communication network and for outputting digital information.
This kind of apparatus comprises a copying machine, a printer and a facsimile machine.
FIG. 9 is a diagram schematically showing essential portions of a digital printer, which is an example of this kind of conventional image forming apparatus.
A photosensitive drum 1 provided as an image bearing member is formed of a cylindrical electroconductive base member and a photoconductive layer formed on the base member, and is supported so as to be rotatable in the direction indicated by the arrow A in FIG. 9.
Around the photosensitive drum 1 are successively disposed, along the direction indicated by the arrow A, a primary charger 2 which uniformly charges the surface of the photosensitive drum 1, an exposing device 3 which reads an original to obtain an image signal according to the density of an image and exposes the photosensitive drum 1 on the basis of the image signal to form an electrostatic latent image, a developing device 4 which attaches toner (developer) to the electrostatic latent image to form a toner image (developed image), a transfer-separation charger 5 which transfers the toner image formed on the photosensitive drum 1 onto a sheet S and separates from the photosensitive drum 1 the sheet S having the transferred toner image on its surface, a cleaning device 6 which removes residual toner on the photosensitive drum 1 after transfer of the toner image, and a pre-exposing device 7 which eliminates residual charge on the photosensitive drum 1.
Sheet S having the transferred image on its surface is transported to a fixing device 8 after being separated from the photosensitive drum 1. In the fixing device 8, the toner image on the surface of sheet S is fixed and the desired printed image is formed by image forming means to be delivered to a place outside the image forming apparatus.
In a reader unit 91, an original placed on an original glass stand 911 is irradiated with light emitted from an illumination lamp 912, and reflected light from the original is imaged on a single-line photoelectric element array 913 provided as a photoelectric conversion element to be converted into an electrical signal in accordance with image information. Reflected light from the original irradiated with light from the illumination lamp 912 is guided by mirrors 914a and 914b and imaged on the photoelectric conversion device 913 by a lens 916. The electrical signal output from the photoelectric conversion device 913 undergoes analog to digital conversion in an analog to digital (A/D) converter 915 to be converted into an 8-bit digital image data. This image data undergoes logarithmic conversion in a black signal generation circuit 917 to convert luminance information into density information, thereby obtaining image density data.
Eight-bit digital image data formed as described above is supplied to a laser drive circuit 301, which is a well-known pulse-width modulation (PWM) circuit. The drive circuit 301 modulates the emission time of a semiconductor laser 304 according to the supplied image density signal so as to change the area grayscale level in each pixel area, thereby realizing a tint or tone.
Laser driving methods are generally grouped into those using a PWM circuit corresponding to that described above and those using a binary laser drive circuit. The PWM circuit modulates, according to the level of the input image density signal, a pulse width signal corresponding to the time period during which the semiconductor laser emits light, as described above. On the other hand, the binarizing circuit converts the image signal into a two-step signal consisting of particular emission-on and emission off signals and inputs this two-step signal to the laser drive circuit 301, thereby turning on and off the semiconductor laser device 304.
Laser light emitted by the above-described laser driving according to the image signal is led to the photosensitive drum 1 by a polygon mirror scanner 302 rotating at a high speed and by a mirror 303 to perform raster scan writing on the photosensitive drum 1, thereby forming a digital electrostatic latent image as image information.
A number of electrophotographic methods, including the inventions patented as U.S. Pat. No. 2,297,961, Japanese Patent Publication (Kokoku) Nos. 42-23910 and 43-24748, etc., are known. Ordinarily, an electrical latent image is formed by one of various available means on a photosensitive drum provided as a recording medium using a photoelectric material and is developed by using toner (developer), and the obtained toner image is transferred onto a recording member such as paper selected as desired and is fixed on the recording member by heating or processing using a solvent vapor or the like to produce an image output.
Also, various development methods for visualizing an electric latent image by using a developer are known, which are, for example, magnetic brush development methods relating to U.S. Pat. No. 2,874,063, powder cloud methods relating to U.S. Pat. No. 2,221,776, fur brush development methods, and liquid development methods. In particular, among these development methods, magnetic brush development methods using a two-component developer having toner and a carrier as main components have been put into wide practical use. This kind of method makes it possible to obtain a good image with comparatively high stability, but entails drawbacks relating to the two-component developer, i.e., degradation of the carrier and variation in the toner/carrier mixture ratio.
To avoid these drawbacks, various development methods using a monocomponent developer composed of toner alone have been proposed. This kind of development method eliminates the need for control of the amount of toner with respect to the toner/carrier mixture ratio and has the advantage of simplifying the image forming apparatus.
Such a monocomponent development method is carried out in such a manner that a developer bearing member 41 of the developing device 4 is disposed so as to be opposed to the photosensitive drum 1 for bearing an electrostatic latent image in a noncontact relationship therewith, and a developing bias voltage E from a power supply 49 is applied between the developer bearing member 41 and the photosensitive drum 1 to perform development of the electrostatic latent image on the photosensitive drum (image bearing means) 1 (see FIGS. 6 and 7).
Methods which have been widely used as a method for forming a developer layer on the surface of the developer bearing member 41 to perform the above-described development are a method in which a member 43 in the form of a plate is used as a developer layer forming means and a suitable pressure xe2x80x9cPxe2x80x9d is applied to the member 43 to maintain the same in contact with the developer bearing member 41 (FIG. 6), and a method in which a developer layer forming member 43 is disposed with a suitable gap G1 formed between the member 43 and the developer bearing member 41 (FIG. 7) and a developer is fed to the vicinity N of the plate-shaped member 43 by rotation of the developer bearing member 41 in the direction indicated by the arrow B. The latter method is suitable for a high-speed image forming apparatus.
Ordinarily, the peripheral speed Vdahm of the developer bearing member is set higher, more specifically 1.4 to 2.2 times higher than that of the image bearing member.
Under the background of the public""s increasing awareness of energy saving in recent years, the proportion of users who perform printing on only one side of toner image bearing media, i.e., printing sheets, in a conventional manner is decreasing while the proportion of users who perform printing on two sides of printing sheets is increasing.
As a dominant system for continuously performing printing on two sides of a plurality of printing sheets (which operation hereinafter referred to as xe2x80x9ctwo-side continuous printingxe2x80x9d), a system has been used in which transfer and fixation are first performed continuously on one side of each of a plurality of sheets (which operation hereinafter referred to as xe2x80x9cone-side printingxe2x80x9d), the sheets on which one-side printing has been performed are temporarily stacked on a stacking means 921 capable of stacking a plurality of sheets, and transfer and fixation are continuously performed on the other sides of the sheets stacked on the stacking means 921 simultaneously or approximately simultaneously with the completion of one-side printing on all the predetermined number of printing sheets or after passage of a predetermined time period (this system hereinafter referred to as xe2x80x9cstacking systemxe2x80x9d) (FIG. 14).
The above-described stacking system, however, requires a space for stacking means in or outside the image forming apparatus and requires setting of a procedure in which recording medium sheets on which one-side printing has been performed are temporarily stacked on the stacking means 921 before one-side printing on all the recording medium sheets is completed. Therefore there is a limit to the number of sheets on which two-side image formation is continuously performed on one printing instruction, and there is also a limit to the reduction in the time required for two-side image formation. For this reason, it is difficult to meet requirements recently made for a higher image formation speed as long as the above-described stacking system is used.
A system for two-side continuous printing, different from the above-described stacking system, has therefore been proposed in which continuous printing is performed on a plurality of sheets in the order of arrival at a transfer region Ztr formed between the photosensitive drum 1 and the transfer device 5 from either of a sheet feeding means such as a sheet feed cassette for feeding a sheet toward the transfer region Ztr and a surface reversing means 922 for retransporting one of the sheets having a toner image transferred onto and heat-fixed on its one surface to the transfer region Ztr after reversing the front side and the back side to enable transfer onto the other surface. (This system will hereinafter be referred to as xe2x80x9cthrough-pass systemxe2x80x9d) That is, the sheet having a toner image fixed on its one surface is transported to the transfer region immediately after being reversed. In this system, when two-side image formation is performed continuously on one printing instruction, either of two, or a mixture of two different kinds of sheets, i.e., one passed through the fixing device and one supplied from the feed cassette and having no transferred toner image (not passed through the fixing device), is supplied to the transfer region. In contrast with the stacking system, this through-path system is free from the above-described restrictions and makes it possible to reduce the size of the image forming apparatus and to increase the two-side image formation speed.
The above-described conventional art, however, entails a problem described below.
Image forming apparatuses based on the above-described conventional art, particularly a high-speed digital type of image forming apparatuses have a problem that deterioration of various image qualities, typically a reduction in image density occurs in the course of continuously outputting images onto two surfaces of a plurality of sheets while minimizing the distance between the sheets successively supplied (minimizing sheet feed intervals).
FIGS. 11 and 12 and Table 1, respectively, show changes in plate inner portion temperature Tdbr, image reflection density Drmax, etc., after two-side outputting of one million pages with A4-size images of a standard image proportion (6%) at a high temperature and high humidity (30xc2x0 C., 80%RH, hereinafter, the same).
A leading cause of these changes is considered to be a process in which the temperature in the vicinity of the developing device is increased by heat caused by eddy current produced by high-speed rotation of the developer bearing member 41 in a case where a magnetic field generation means is incorporated and/or by heat accumulated by successive passage through the transfer region of sheets fed for toner image transfer on the second side of the sheet after being heated by the heat-fixing device, the sheets being fed at such small intervals that the temperature of the sheet is not lowered before the sheet enters the transfer region; the temperature of the plate-shaped member 43 and the sleeve surface are thereby increased; and the developer having its layer thickness regulated (rubbed) between the plate-shaped member 43 and the development sleeve is affected by the heat, that is, degradation and a reduction in chargeability of the developer composed mainly of a resin are caused by the heat. It is also considered that as the developer takes up moisture, degradation and a reduction in chargeability of the developer occur.
It can be understood from FIGS. 11 and 12 and Table 1 that when the temperature Tdbr of the atmosphere in the apparatus or the absolute water amount xcex7aw in the atmosphere in the apparatus is increased, the image reflection density Drmax or the like becomes reduced and other image qualities become worse.
xe2x80x9cImage reflection density Drmaxxe2x80x9d referred to in this specification is the average of values obtained by measuring, with a reflection densitometer, a product RD-914 (trademark) from MacBeth Corp (USA), five points in a copied image corresponding to circular original image portions having a reflection density of 1.2 and a diameter of 5 mm.
Also, xe2x80x9cfoggingxe2x80x9d referred to in this specification is represented by a value obtained by subtracting from the reflectivity of unused paper the average of the reflectivities of measurement-object paper at nine white points after image formation, measured with a reflection densitometer, a product TC-6DS (trademark) from Tokyo Denshoku Co.
Today, with the development of network environments, etc., there is a need for further increasing the image outputting speed and the amount of copies and the above-described problem is becoming more serious.
To prevent an increase in the temperature of the plate-shaped members, a Peltier element (also called a semiconductor heat pump, a device which becomes cooler at its one side and becomes hotter of its the other side when supplied with current, and which is used for temperature compensation in a temperature-sensitive device such as a semiconductor laser) may be used. However, such a device is high-priced and requires a fan or the like for generating air flow for removing heat radiated from the high-temperature side.
In two-side printing performed with growing frequently as mentioned above, the sheet having a high temperature (e.g., 80xc2x0 C.) immediately after passage through the fixing device 8, i.e., immediately after the completion of printing on the first side passes through the transfer section Ztr again without a pause or detour for cooling, and heat radiated from the sheet increases the temperature in the vicinity of the developing device.
In the through-path system, a temperature reduction effect similar to that in the case of the stacking system where the temperature of sheets having fixed toner images on the first surfaces is reduced while the sheets are accumulated on the intermediate tray cannot be expected. For this and other reasons, the sheet temperature immediately after the completion of printing on the first side is much higher than that in the system using the intermediate tray, and the increase in temperature in the vicinity of the developing device is considerably higher (FIG. 13).
Further, if the relative humidity is higher, i.e., the absolute water amount is high while the atmospheric temperature is constant, the influence on image quality (a reduction in image density) is larger (Table 1).
In view of the above-described circumstances, an object of the present invention is to provide an image forming apparatus capable of forming high-quality images on two surfaces of recording mediums with stability during a long period of time.
Other objects of the present invention will become apparent upon reading the following detailed description of the invention.